add GeometryField and features api endpoint

src/c3nav/mapdata/fields.py

@@ -0,0 +1,17 @@
+import json
+
+from django.db import models
+from shapely.geometry import mapping, shape
+
+
+class GeometryField(models.TextField):
+    def from_db_value(self, value, expression, connection, context):
+        if value is None:
+            return value
+        return shape(json.loads(value))
+
+    def to_python(self, value):
+        return shape(json.loads(value))
+
+    def get_prep_value(self, value):
+        return json.dumps(mapping(value))